PROJECT SUMMARY/ABSTRACT Proper establishment of jaw length is essential to feeding, breathing, and normal development of oral-motor skills. The jaw often displays an array of size related abnormalities, including mandibular hypoplasia, retrognathia, asymmetry, and clefting. Treatment for craniofacial defects often involves multiple surgical interventions, which is a lengthy, costly, and emotionally and physically draining process. Prevention for at-risk individuals, however, provides a welcome alternative. Identifying potential strategies for rescue or regeneration depends on understanding the developmental processes regulating jaw size. The jaw, along with most of the craniofacial skeleton, derives from the neural crest mesenchyme (NCM), a transient multi-potent cell population that arises at the neural plate border. We employ a unique in vivo system to study the NCM and how it executes molecular and histological programs that establish the size and shape of the jaw skeleton. Understanding how the NCM accomplishes such a complex task, and the specific mechanisms that determine jaw length, remain unknown. Our published and preliminary results suggest that differential regulation of matrix metalloproteinases (Mmps), Runx2, and Transforming Growth Factor-Beta (TGF?) in a species-specific manner may contribute to differences in jaw length. We hypothesize that the NCM differentially regulates Mmps, Runx2, and TGF? signaling in a species-specific manner, modulating bone resorption to generate variations in jaw length. To test our hypothesis, we combine the species-specific developmental programs of quail and duck in a novel chimeric system. Quail have short jaws whereas those of duck are relatively long, and quail embryos develop much faster than duck. Exchanging NCM between quail and duck provides a unique way to manipulate signaling between donor NCM and adjacent host tissues, and allows discovery of NCM- dependent processes. In Aim 1, we will determine the extent to which the NCM-mediates Mmps to regulate jaw size. In Aim 2, we will ascertain whether jaw size is affected by differential regulation of NCM-mediated Runx2 expression. In Aim 3, we will determine the extent to which NCM mediates jaw length through TGF? signaling. We will employ gain- and loss-of-function strategies to understand how changes in bone formation and resorption, and how TGF? can regulate Runx2 and Mmp expression to affect jaw length. Each Specific Aim is clinically relevant and may identify molecular therapies that can be used to manipulate jaw length. We are confident that our research will provide a foundation for biologically-based, non- surgical methods to treat disorders of the human jaw.